I don't recall offhand whether you can do that directly in Audition CS5.5, but in the version about to be released (CS6) there's a menu option that will split the current stereo file open in Waveform view into two mono ones, and since these are automatically placed in the available files menu, they can be directly dropped into Multitrack view.

This is an important inclusion because many location recorders save multitrack audio as single multichannel .wavs. This allows them to be split into individual multitracks in Audition to be worked on.

I'm rather spoiled from that point of view. There's a 3rd party app for my Alesis HD24XR/Fireport which lets me save any tracks in whatever format I like, so I can produce multichannel wav files as well as mono and stereo ones, from any combination of tracks I choose. Where I find the mono splitter most useful is with M-S stereo files. I usually put a M-S pair in the middle (forward) section of my Decca Tree, and if I ever need to use it as such, it makes sense to have it in a manipulatable form for plugins to operate on, so I create a stereo file. But more often than not, I don't need to do this, and just want the M part of it as a centre mic for the Tree. So being able to easily whip that out as a mono file, and have it panned dead centre is extremely useful, and I can still retain the original file. Did it this very evening, in fact...